Non-contact electromagnetic tracking systems are well known in the art, with a wide range of applications.
Non-contact tracking systems have been described based on electromagnetic, optical and sonographic detection methods. In many applications, electromagnetic tracking is preferred, because unlike other methods, there need not be a clear line of sight between the object and the detectors. Electromagnetic tracking can also be made relatively immune to interference from background signals, thus avoiding problems such as those caused by stray light in optical systems. Furthermore, the components required for electromagnetic tracking are easily produced and inexpensive.
Exemplary prior art patents for electromagnetic tracking systems include U.S. Pat. Nos. 3,046,228 and 3,121,228, which describe a tracking system for measuring the position of an object in a plane. This system uses two orthogonal coils to generate a rotating dipole field in the vicinity of the object, thereby causing electrical currents to flow in two orthogonal sensor coils. Measurement of the sensor coil current signal is used to determine the object's position.
U.S. Pat. Nos. 3,868,565, 3,983,474 and 4,017,858 describe a tracking system using three orthogonal coils to generate a dipole field, which nutates around a direction vector. The object's position is determined by measuring the current signals generated in three orthogonal sensor coils.
U.S. Pat. No. 4,054,881 describes a tracking system using three coils to generate electromagnetic fields in the vicinity of the object. The fields generated by these three coils are distinguished from one another by open loop multiplexing of time, frequency or phase. The signal currents flowing in three orthogonal sensor coils are used to determine the object's position, based on an iterative method of computation.
U.S. Pat. No. 4,314,251 describes a variation on the '881 patent, wherein only five coils are needed in total, and a non-iterative method of computing is used.
U.S. Pat. No. 4,287,809 describes a tracking system with two or more coils to generate electromagnetic fields, using time multiplexing to distinguish between them. Three orthogonal sensor coils are used to determine the object's position.
Finally, U.S. Pat. No. 4,849,692 describes a tracking system with three orthogonal coils to generate periodic DC electromagnetic fields. A receiver measures the DC field at the object to determine its position.
It would be desirable to provide an electromagnetic tracking method whereby the object or objects being tracked have no physical connection to the surroundings. Desirably, such a method would require that only passive components be mounted on the object, and the object's motion would be unrestricted within the field of detection. All power, active components and additional signals conveyed from the transmitter to the sensor and signal processing circuits would preferably be restricted to the fixed portion of the tracking system.